How the inverter works

Abstract

The present invention relates to a control method for operating an inverter in an energy grid connected to the energy grid via a controllable disconnect switch. The method includes operating the inverter in a current-applying mode while the disconnect switch is closed and constantly monitoring a voltage drop in the power grid. If a voltage drop in the power grid is detected, the method changes the operating mode of the inverter to a voltage-setting mode, in which the inverter sets an interim voltage that is reduced relative to the grid's normal voltage. If the energy grid voltage drop persists after a predetermined time has elapsed after the voltage drop has been detected, the method includes opening the disconnecting switch to increase the voltage set by the inverter to the grid's normal voltage, and if the energy grid fault does not persist after a predetermined time has elapsed after the voltage drop has been detected, operating the inverter in the current-applying mode.

Description

[Technical Field]

[0001] The present invention relates to a method for operating an inverter and to an energy system. [Background technology]

[0002] Energy generation from fossil fuels has been declining for the past few decades. It is much more preferable to generate energy, especially electrical energy, from renewable sources. Electrical energy from renewable sources is provided by the energy source in the form of direct current, which must first be converted into alternating current by an inverter before it can be used by the producer itself or fed into the energy grid, especially the AC grid.

[0003] "Different faults can occur in the energy grid, and the inverter reacts to these. For example, if a short circuit fault occurs in the energy grid, the standards stipulate that the inverter must supply the maximum possible power to the energy grid. In other cases, a fault may occur in the energy grid, making it impossible to supply or draw power from the grid."

[0004] The prior art knows methods that can be used to detect the above errors. However, it is desirable that the partial grid, on which both the energy source and the at least one inverter are located, continue to be supplied with power. The prior art does not cover the transition from supplying the partial grid with the energy supply grid to supplying the partial grid as an island grid after being disconnected from the energy supply grid.

[0005] DE102019116254A1 shows a method for switching between a current-controlled and a voltage-controlled operating mode of an inverter.

[0006] Furthermore, so-called STATCOMs are known to stabilize energy supply grids. Summary of the Invention

[0007] A method for operating an inverter in a partial grid connected to an energy supply network via a controllable disconnect switch according to the invention comprises the steps of: operating the inverter in a force current mode while the isolation switch is closed; continuously monitoring a voltage drop in the power grid; If a voltage drop in the energy supply grid is detected, changing the operation mode of the inverter to a voltage set mode, wherein a temporary voltage that is lower than the grid normal voltage is set by the inverter; If the voltage drop in the energy supply network persists after a specified time has elapsed since the grid fault was detected, opening the disconnecting switch to continuously increase the applied voltage to the grid normal voltage; If the energy grid does not resume operation after a specified time has elapsed since the grid fault was detected, operating the inverter in a force current mode. Equipped with.

[0008] The method controls inverters in an energy grid, also known as partial grids, that can be disconnected from the energy grid and are disconnected under certain conditions, including failure of the energy grid's supply capacity in the sense that the energy supply of the partial grid is no longer guaranteed by the energy grid, in other words, it is no longer possible to obtain power from the energy grid.

[0009] In particular, in this context, the inverter, which initially operates in current-applied mode, must generate a voltage in the partial grid when it is disconnected from the energy supply network, ie in voltage-applied mode.

[0010] In the current application mode, the inverter synchronizes with the grid voltage of either the energy supply grid or the partial grid and adjusts the supplied current. Therefore, as long as the power supply grid supplies energy to the partial grid, i.e., as long as the disconnecting switch between the partial grid and the power supply grid is closed, the inverter can synchronize with the power supply grid and supply power to the partial grid according to the frequency, voltage, and phase of the power supply grid.

[0011] In voltage application mode or voltage setting mode, the inverter detects insufficient or no grid voltage from the energy supply or sub-grid. If the method detects a voltage drop in the sub-grid during current application mode, i.e., while the disconnect switch to the energy supply is closed, the inverter switches to voltage application mode. The inverter then independently regulates the sub-grid voltage according to standard specifications for voltage level and frequency. In Central Europe, the standard frequency is 50 Hz and the standard voltage is 230 V. However, these specifications may vary depending on the grid operator at the regional level.

[0012] The voltage drop can be determined by any means of measuring voltage. For example, a voltmeter can be connected to the line between the energy supply network and the disconnect switch. This means that the voltage provided by the energy supply network just before the disconnect switch, i.e., at the connection point to the partial grid, can be measured directly. Alternatively, a voltmeter can be connected to the AC input of the inverter. If the built-in voltmeter detects a voltage drop in the energy supply network, the inverter can switch its operation from current-forcing mode to voltage-setting mode without delay, for example by signal transmission from an external voltmeter.

[0013] In one embodiment, a voltage drop in the energy grid is detected by a voltage drop in the energy grid of a threshold value, preferably 5%.

[0014] In other words, a voltage drop is detected when the voltage of the power grid drops by at least a threshold, preferably 5%, compared to the grid normal voltage. This involves comparing the current voltage of the power grid with the grid normal voltage, which is the nominal voltage specified for the local or regional power grid (230V in Europe).

[0015] This embodiment offers several advantages when detecting voltage drops: Even small voltage drops in the energy grid can be detected. This means that gradually decreasing voltages can be detected early, minimizing delays in switching the inverter between current and voltage application modes. The creation of island grids within the partial grid and the inverter's power supply to the partial grid can be performed with almost no delay. As a result, consumers can enjoy an almost uninterrupted power supply.

[0016] If the voltage drop is simply a fluctuation in the energy grid voltage and the energy grid returns to the normal grid voltage after the fluctuation ends, the method ends, but if there is no recovery as described above and the energy grid voltage gradually decreases, the inverter is already in voltage setting mode and can take over the energy supply of the partial grid.

[0017] In one embodiment, the control method can be designed so that the voltage supplied by the inverter is of the same frequency and phase as the voltage before the voltage drop in the energy grid is detected. This embodiment particularly includes, but is not limited to, when the energy grid returns to normal operation after a voltage drop. This means that the voltage drop is a temporary effect that will then be improved by the energy grid voltage returning to its nominal value. This embodiment facilitates the transition to normal operation, as will be described in more detail below.

[0018] When a voltage drop occurs in the energy grid, the inverters of the sub-network switch from current-feeding mode to voltage-setting mode, as previously described. In voltage-feeding mode, the voltages the inverters inject into the sub-grids are at the same frequency and in phase as the voltages before the voltage drop in the energy grid was detected. When the energy grid becomes available again, the inverters do not readjust their voltage phases because the energy grid and the sub-grid are in phase. This allows for a quick and easy switch back to normal operation, with the energy grid supplying power to the sub-grids.

[0019] In one embodiment, the control method can be designed so that the voltage provided by the inverter drops by 10% to 30% compared to the grid standard voltage of the energy supply network.

[0020] In one embodiment, the control method can be designed to, when monitoring the partial grid to detect a voltage drop in the energy supply network, if it detects that the grid fault is due to an uncorrectable short circuit error, adjust the temporary voltage set by the inverter to the voltage last detected in the partial grid before the voltage drop was detected and immediately open the disconnect switch.

[0021] This embodiment particularly addresses uncorrectable short-circuit errors in the energy grid, which may be caused by a short circuit on a nearby line of a partial grid. To prevent the energy grid in an environment from collapsing, there is a normative requirement that, when a short-circuit error occurs, the inverter continues to supply power to the energy grid for a certain period of time, as long as possible and necessary. In this case, the inverter detects a short-circuit error in the energy grid, where the voltage of the energy grid suddenly drops to a low value, but the residual voltage of the energy grid is still measurable. For example, the voltage of the energy grid drops by 50%. If the short-circuit error is only temporary, the inverter can support the energy grid so that the temporary short-circuit error does not cause the energy grid to completely fail.

[0022] If an uncorrectable short-circuit error occurs, the energy grid will not recover after a certain time. This means that even further power supply by the inverter of the partial grid cannot restore the energy grid. To prevent the partial grid from also failing, the control method of the present invention provides for switching the inverter to voltage setting mode and immediately disconnecting the partial grid from the energy grid. Disconnection from the energy grid is achieved, for example, via a disconnection switch located between the energy grid and the partial grid. If a short-circuit error occurs in the energy grid, the inverter can distinguish between a correctable short-circuit error, which is no longer relevant once the energy grid has recovered, and an uncorrectable short-circuit error, which will not be corrected within a specified time. Furthermore, the inverter can distinguish between a short-circuit error and a grid fault in that, in the event of a grid fault, the measurable voltage on the energy grid drops to 0 V, i.e., there is no measurable voltage on the energy grid.

[0023] In one embodiment, the control method can be designed such that, when monitoring the partial grid to detect a voltage drop in the energy grid, if it is detected that the voltage drop in the energy grid is caused by a fault in the energy grid, the temporary voltage set by the inverter is adjusted to 90% of the voltage last detected before the fault in the energy grid is detected, and the disconnecting switch is immediately opened.

[0024] An aspect of the invention relates to an energy system having at least one inverter configured to be controlled according to a method according to any of the preceding claims and an isolation switch capable of disconnecting the power system from an energy supply grid. [Brief explanation of the drawings]

[0025] The present invention will now be described with reference to the accompanying drawings. [Figure 1] FIG. 1 is a flow chart of a control method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] The figures are explained in detail below.

[0027] FIG. 1 is a flowchart of a control method according to the present invention. The method begins with step S100, which marks the start of the method. In most cases, this is accomplished by starting up the inverter on which the control method is implemented. The method then proceeds to step S110. In step S110, the inverter operates in a current-feed mode. In current-feed mode, the inverter is synchronized to the voltage of the energy grid and supplies power from a connected DC source to the partial grid to which the inverter is connected. The energy grid is connected to the partial grid to which the inverter is connected via a disconnect switch. For example, a voltmeter that continuously measures the voltage of the energy grid can be connected to the disconnect switch, which is permanently connected to the energy grid. The method then continues with step S120.

[0028] In step S120, the voltage drop of the energy supply network is continuously monitored. For this purpose, the voltage of the energy supply network is continuously measured by a voltage measuring device such as a voltmeter. If a voltage drop is detected in step S130, the inverter switches to voltage setting mode. The set voltage is compared with the grid normal voltage of the energy supply network and drops by 10 to 30% of the grid normal voltage.

[0029] The following is the case when the voltage drop persists. If the voltage drop with residual voltage persists in the energy supply network, after a specified time, it is determined that an uncorrectable short-circuit error exists. In this case, it is assumed that the energy supply network has failed. In the case of a power outage, i.e., if the voltage can no longer be measured in the energy supply network, it is assumed that the voltage drop will continue as soon as the power outage is detected. The method proceeds to step S140 via decision Y in step S130.

[0030] In step S140, the disconnecting switch between the energy supply network and the partial grid is opened, i.e., the power transmission connection between the energy supply network and the partial grid is interrupted. The inverter operating in voltage setting mode supplies power to the partial grid, and the consumers in the partial grid operate using this supplied power. The inverter, whose supply voltage has dropped compared to the grid normal voltage of the energy supply network, increases the supply voltage to the grid normal voltage of the energy supply network.

[0031] In another case, the voltage drop in the energy grid voltage decreases. In other words, the energy grid voltage rises from the temporary drop due to the temporary error to the normal grid voltage, and the energy grid voltage no longer drops below the threshold. In this case, the method continues via N in S130, and the method continues to step S110, i.e., the inverter again operates in the current-applied mode. After this mode change of the inverter, the process again proceeds to step S120, where the inverter continues to continuously monitor the energy grid.

[0032] Reference symbol list S100-S150 Step

Claims

1. 1. A control method for operating an inverter in an energy grid connected to an energy supply network via a controllable disconnect switch, comprising: operating the inverter in the current application mode while the isolation switch is closed (S110); continuously monitoring a voltage drop in the power grid (S120); If a voltage drop of the energy supply grid is detected, changing the operation mode of the inverter to a voltage setting mode (S130), in which a temporary voltage dropped from a grid normal voltage is set by the inverter; If the voltage drop in the energy supply grid persists after a predetermined time has elapsed since the voltage drop was detected (Y in S130), opening the cutoff switch to raise the voltage supplied by the inverter to the grid normal voltage (S140); If the fault in the energy supply network does not persist after a set time has elapsed since the voltage drop was detected (N in S130), operating the inverter in a current application mode (S110); A method comprising:

2. 2. The control method according to claim 1, 10. The method of claim 9, wherein a voltage drop in the energy grid is detected by a voltage drop in the energy grid by a threshold value.

3. 3. The control method of claim 2, wherein the threshold is at least 5% of the grid normal voltage.

4. 4. A control method according to claim 1, wherein the voltage supplied by the inverter is selected at the same frequency and phase as the voltage before a voltage drop in the energy supply network is detected.

5. 4. The control method according to claim 1, 2 or 3, The method, characterized in that the reserve voltage provided by the inverter is reduced by 10% to 30% compared to the grid normal voltage of the energy supply network.

6. 6. A control method according to claim 1, characterized in that when monitoring the partial grid to detect a voltage drop in the energy supply network, it is detected that there is an uncorrectable short circuit error, the temporary voltage supplied by the inverter is adjusted to the last detected voltage before the fault in the energy supply network was detected, and the disconnecting switch is immediately opened.

7. 6. A control method according to any one of claims 1 to 5, characterized in that, when monitoring the partial grid to detect a voltage drop in the energy supply network, a fault in the energy supply network is detected, the temporary voltage supplied by the inverter is adjusted to a voltage that is 90% of the last detected voltage before the fault in the energy supply network was detected, and the disconnecting switch is immediately opened.

8. An energy system comprising at least one inverter configured to be controlled according to the control method of any one of claims 1 to 7, and a disconnect switch capable of disconnecting the energy system from an energy supply grid.

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